Regulations of the Ministry of Home Affairs, Japan describe hydrolyzed protein-based foams, synthetic detergent foams and aqueous film-forming foams as three types of fire-extinguishing foams. The protein-based foams exhibit by far the best effect against fires of petroleums, especially conflagration of oil tanks.
Conventional protein-based fire-extinguishing foams produced in Japan start from meals (keratin) of hoofs and horns of horses and bovines as raw materials. For example, Japanese Patent Publication No. 7896/57 discloses a method for producing such a fire extinguisher, which comprises adding sodium hydroxide and water to a hoof and horn meal, heating the mixture at about 100.degree. C. for several hours to hydrolyze proteins partially, adding an inorganic acid such as hydrochloric acid or sulfuric acid or an organic acid such as oxalic acid or acetic acid to neutralize it, removing insoluble substances, concentrating the residue to a suitable concentration, and incorporating additives for imparting oil resistance, heat resistance and foam stability, for example by adding several percent of a ferrous salt such as ferrous sulfate or ferrous chloride, an antiseptic such as benzoic acid or pentachlorophenol sodium salt, and an anti-freezing agent such as ethylene glycol or propylene glycol.
Since the hoof and horn meal used as a raw material in the above method is a scleroprotein, its hydrolysis requires a period of several hours, and at times, it is necessary, as disclosed in Japanese Patent Publication No. 3643/64, to heat-treat it at 80.degree. to 130.degree. C. for 1 to 8 hours prior to hydrolysis. Furthermore, since hoof and horn meals contain large quantities of sulfur-containing amino acids, they give off marked offensive odors during alkali decomposition, and the final decomposition products also smell extremely bad. Since the hoof and horn meals are naturally occurring substances most of which are imported from South East Asia and China, the supply of these goods is unstable both in cost and in quantity, and their quality also varies greatly.
In recent years, single-cell organisms or microbial cells which can be produced commercially in large quantities have attracted attention as protein sources that will replace the hoof and horn meals. Methods have already been proposed to produce fire-extinguishing foams and other types of foams by hydrolyzing microbial cells with an alkali such as calcium hydroxide or sodium hydroxide. This alkali hydrolysis is usually performed by adding 15 to 50% by weight of calcium hydroxide or sodium hydroxide to the microbial cells, adding water to a cell concentration of 10 to 30%, and heating the mixture at 100.degree. C. for at least several hours.
When microbial cells are decomposed with alkalies, however, their organic ingredients such as lipid, carbohydrates, and nucleic acids and their related substances, and their inorganic ingredients such as phosphorus, potassium and calcium, which are present in amounts of several percent to several tens of percent, present various difficulties.
When only sodium hydroxide is used as an agent for hydrolysis, a ferrous salt which is added to the filtrate left after the removal of the alkali decomposition residue is almost entirely precipitated and wasted, and the hydrolyzate gels on cooling. To avoid these difficulties, Japanese Laid-Open Patent Publication No. 31698/75 suggests the hydrolysis of microbial cells after heat-treating them in an acid aqueous solution. This method can prevent the formation of a precipitate at the time of neutralization and of adding a ferrous salt, but since the hydrolyzate still gels, it should be further treated to remove lipid.
Accordingly, in order to hydrolyze microbial cells using sodium hydroxide alone, a chemical treatment such as heat-treatment in acidity and a lipid-removing treatment are required, and this method cannot be applied to untreated microbial cells or to cells subjected to enzymatic treatment or a physical treatment. Furthermore, in the hydrolysis of microbial cells heat-treated in an acid aqueous solution using sodium hydroxide, the sodium hydroxide used is converted to a salt of an organic acid such as sodium oxalate or a salt of an inorganic acid such as sodium chloride or sodium sulfate by the subsequent neutralization treatment, and remains in the resulting fire-extinguishing foams. As a result, the concentration of salts in the fire-extinguishing foams increases, and the concentration of the protein hydrolyzate as a main ingredient decreases relatively. This adversely affects the performance of the fire-extinguishing foams, especially their foaming properties.
On the other hand, the present inventors have found that when only calcium hydroxide is used, untreated microbial cells are scarcely hydrolyzable even if heat-treated for long periods of time. It is difficult to hydrolyze the cells even if 30% by weight of calcium hydroxide and 500% by weight of water are added to the microbial cells, and the mixture is heat-treated at 100.degree. C. for 10 hours. When as disclosed in Japanese Laid-Open Patent Publication No. 31698/75, the microbial cells are hydrolyzed after they have been heat-treated in an acid aqueous solution (pH not more than 1.5), the apparent volume of the solution during decomposition expands by foaming to 2 to 3 times to make its stirring difficult. Moreover, the amount of the alkali hydrolysis residue is large, and the yield of the protein hydrolyzate in the filtrate after removal of the residue is reduced.
It is an objective of this invention to provide a process for hydrolyzing microbial cells which removes the defects of the aforesaid conventional techniques, hydrolyzes proteins in the microbial cells rapidly and uniformly at high conversions, remove other ingredients of the cells, such as lipid, phosphoric acid, and nucleic acids and their related substances, is simple in post-treatment after hydrolysis, and can be suitably used to produce hydrolyzed protein-based fire-extinguishing foams, concrete foaming agents, and other types of foaming agents.
The present invention is based on the discovery that when microbial cells are subjected to a hydrolyzing treatment by using a combination of an alkaline earth metal hydroxide and an alkali metal hydroxide, most of the lipid, phosphoric acid, nucleic acids and their related substances move to the alkali hydrolysis residue, and scarcely remain in the filtrate after removal of the residue.